The structure and expression of deletion-type and full-length bovine leukemia virus (BLV) proviruses have been examined-from tumors of infected cattle, sheep, and established cell lines. A deletion-type provirus present in a bovine lymphoid cell line (NBC-13) has been cloned, sequenced, and shown to arise by a recombination event between two 10 base-pair direct repeats. Different deletion-type proviruses were detected in both tumors and circulating lymphocytes from infected pre-tumor animals. These deletion-type proviruses are different in size, but maintain the pX region of the genome. One of these proviruses, from tumor 85 X 1007, has been molecularly cloned and is currently being analyzed. The transcriptional activity of the deletion-type provirus from NBC-13 is enhanced by growth in cell culture in the presence of horse serum, and is inhibited by factors present in fetal calf serum. Examination of lymphocytes from experimentally-infected sheep shows that the BLV provirus is monoclonal 3 to 6 months prior to the appearance of tumors. A spontaneous amplification of B-lymphocytes occurred in one animal and transcripts originating from the pX region of the provirus were detected.